1. Field of the Invention
The present invention generally relates to local area networks (LANs) employing a token-based ring-of-trees topology and, more particularly, to a method and apparatus for allowing interoperation of a token ring packet data network managed by a timed-token protocol (Fiber optic Distribution Data Interface-I or FDDI-I) with a time division multiplexed network which allocates a portion of its bandwidth to the packet data timed-token protocol (Fiberoptic Distribution Data Interface-II or FDDI-II).
2. Description of the Prior Art
The interconnection and interoperation of distributed computing devices is an old but continuing challenge. In recent years many of those who have thought about this challenge have concluded that the most effective way to promote progress is to parse the task into relatively independent but connected pieces and develop a corresponding set of standards. Different systems can then communicate if they follow the standards at their corresponding levels; they may rely upon parallel coordination through standards at other levels to complete the communication path. The Open Systems Interconnection (OSI) reference model reflects this view.
The OSI model defines seven layers--from the physical link dealing with mechanical, electrical and optical characteristics of signals to the application layer which provides file transfer and network management services to users. Local area network (LAN) standards within the OSI model are the subject matter and constraints for the present invention. Local area networks are characterized by a topology (e.g. ring or bus) for connecting devices to the LAN's signal transmission media (e.g. cable or optical fiber) and a protocol (e.g. token-ring or timed-token) for controlling access to the medium by an attached device.
Communication capacity over a LAN is limited by the transmission medium, and also by the topology and the relevant protocols at both the physical and media access control levels. "Bandwidth" is a measure of the communication capacity of a LAN, and is commonly given in bits per second. For example, the bandwidth of a coaxial cable Ethernet LAN may be 10 million bits per second (Mbps) while a Fiber optic Distributed Data Interface (FDDI) LAN using a timed-token protocol may be 100 Mbps.
However, these figures are nominal capacities. Typically, the aggregate communication needs of devices attached to a LAN are an irregular mix of stream and bursty traffic. The LAN's capacity may be underutilized at one time and overloaded at another.
The FDDI standard did not allow for isochronous packet switched data, such as voice and video. Consequently, an enhanced version of the standard, FDDI-II, was developed. FDDI-II accommodated isochronous data through time division multiplexing of the available bandwidth, allocating a portion of that bandwidth to the timed-token protocol used in FDDI.